Today, plastics material containers which are thermoformed from sheet material are in extensive use for containing foodstuffs such as cream, yoghurt, mousse, ice cream and so on. These containers always have lids but, generally speaking the plastics containers are lidded either with metallic foil or waxed cardboard. The utilization of different materials for the lid and the container is an inconvenience because this usually involves two suppliers.
Many difficulties are experienced in applying metal lids, metallic foil, or waxed cardboard to serve as closures for plastics material containers. For example, heavy sealing forces need to be applied in the case of metal lids and such forces can lead to distortion of the plastics material of the plastics container which in turn can lead to leakage of the contents of the container after the sealing thereof.
Where metal foil is used to seal the container, this suffers from the disadvantage that invariably an adhesive must be used or the metal foil must be pre-coated with a material which will seal to the flange of the container. In any event, metal foil has limited strength to such an extent that a container sealed thereby is not suitable for holding, for example, carbonated beverages. The waxed cardboard lids for these containers suffer from the same disadvantages as the foil lids, and have the additional disadvantage that cardboard is highly permeable to liquids and gases, hence the reason for the wax coatings, and furthermore the step of having to apply the wax coating makes the manufacture of cardboard lids expensive.